Methods and systems for motor control

ABSTRACT

Various embodiments of the present technology comprise a method and system for motor control. Various embodiments of the present technology may comprise a control circuit that computes an adjusted rotational speed to compensate for a clock signal that deviates from an ideal (expected) clock signal value.

BACKGROUND OF THE TECHNOLOGY

Conventional motor systems operate according to a clock signal. Thesystem may use the clock signal to determine the rotational speed of themotor and, in turn, use the clock signal to increase or decrease therotational speed of the motor to reach a target rotational speed. Theclock used to generate the clock signal may introduce errors in theclock signal. Therefore, the resulting clock signal is not accurate and,in turn, control of the motor's rotational speed is affected by theerror in the clock signal.

SUMMARY OF THE INVENTION

Various embodiments of the present technology comprise a method andsystem for motor control. Various embodiments of the present technologymay comprise a control circuit that computes an adjusted rotationalspeed to compensate for a clock signal that deviates from an ideal(expected) clock signal value.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A more complete understanding of the present technology may be derivedby referring to the detailed description when considered in connectionwith the following illustrative figures. In the following figures, likereference numbers refer to similar elements and steps throughout thefigures.

FIG. 1 is a block diagram of a motor system in accordance with a firstembodiment of the present technology;

FIG. 2 is a block diagram of a motor system in accordance with a secondembodiment of the present technology;

FIG. 3 is a first flowchart for operating a motor system in accordancewith various embodiments of the present technology;

FIG. 4 is a second flowchart for operating a motor system in accordancewith various embodiments of the present technology;

FIG. 5 is a third flowchart for operating a motor system in accordancewith various embodiments of the present technology; and

FIG. 6 is a fourth flowchart for operating a motor system in accordancewith various embodiments of the present technology.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present technology may be described in terms of functional blockcomponents and various processing steps. Such functional blocks may berealized by any number of components configured to perform the specifiedfunctions and achieve the various results. For example, the presenttechnology may employ various motors, clocks generators, samplingcircuits, voltage shifters, counters, drivers, and the like, which maycarry out a variety of functions. In addition, the present technologymay be practiced in conjunction with any number of motor systems, suchas a single-phase motor system, a 3-phase motor system, and the like,and the motor systems described are merely exemplary applications forthe technology. Further, the present technology may employ any number ofconventional techniques for sampling data, performing arithmeticfunctions, detecting back EMF (electromotive force), detectingzero-crossing points, and the like.

Methods and systems for motor control according to various aspects ofthe present technology may operate in conjunction with any suitableelectronic system and/or device that uses a motor to convert suppliedelectrical energy into mechanical energy. Referring to FIGS. 1 and 2, amotor system 100 may comprise various components suitable for detectingand/or controlling a rotational speed of a motor 120. For example, themotor system 100 may comprise a clock generator circuit 125 thatoperates in conjunction with a control circuit 105.

According to various embodiments, the motor 120 is configured to convertelectrical energy into mechanical energy. For example, the motor 120 maycomprise a single-phase motor (such as illustrated in FIG. 1) or 3-phasemotor (such as illustrated in FIG. 2). In the case of a single-phasemotor, the motor 120 further comprises a set of coils 185 that areresponsible for controlling the rotational speed of the motor 120 and ahall sensor 180 to generate a hall sensor signal in response to adetected rotational speed.

According to various embodiments, the motor system 100 may comprise afeedback circuit configured to measure and/or detect an actualrotational speed of the motor 120. For example, in the case where themotor 120 comprises a single-phase motor, the motor system 100 mayfurther comprise a comparator 135. A first input terminal and a secondinput terminal of the comparator 135 may be connected to the hall sensor180 to detect an orientation of a rotating part of the motor 120 bymeasuring variations in the magnetic field. The comparator 135 may beconfigured to generate a comparator output signal at an output terminalaccording to output signals from the hall sensor 180. The comparator 135may, in turn, provide position and/or rotational information of themotor 120, according to the output signals from the hall sensor 180,back to the control circuit 105.

In a case where the motor 120 comprises a 3-phase motor, the motorsystem 100 may further comprise a BEMF circuit 205 and a zero-crossingpoint detection circuit 210 that operate in conjunction with each otherto enable position recognition of the motor 120. The BEMF circuit 205may be configured to sense back EMF (the voltage induced into a statorwinding due to rotor movement) of motor 120 and generate a back EMFsignal. The BEMF circuit 205 may comprise any circuit and/or systemsuitable for detecting back EMF.

The zero-crossing point detection circuit 210 may detect thezero-crossing points of the back EMF signal and generate a zero-crossingsignal. The zero-crossing point detection circuit 210 may comprise anycircuit and/or system suitable for detecting zero-crossing points of asignal.

The BEMF circuit 205 together with the zero-crossing point detectioncircuit 210 may, in turn, provide position and/or rotational informationof the motor 120 back to the control circuit 105.

In various embodiments, the clock generator circuit 125 may beconfigured to generate a clock signal CLK with an actual frequency f′and transmit the clock signal CLK to the control circuit 105. The actualfrequency f′ of the clock signal CLK may deviate from an ideal(expected) frequency f. In such a case, the clock generator circuit 125via the clock signal CLK may introduce an error (a difference betweenthe ideal frequency f and the actual frequency f′) into the motor system100. The error may propagate throughout the motor system 100, whichcontributes to errors in the detection of the rotational speed of themotor 120 and inability to control the rotational speed of the motor 120as desired.

For example, in an ideal case, a rotational speed of the motor 120 isrepresented as: R=C/t, where C is a constant, t is an ideal countervalue (when clock frequency is also ideal) according to a counter 150,which corresponds to the length of time for one rotation cycle of themotor 120. The constant C is a predetermined value based on the idealclock frequency f, and characteristics of the motor 120, such as anumber of phases of the motor 120, a number of poles of the motor 120,and a number of slots of the motor 120. However, due to errorsintroduced by the clock signal CLK, the actual clock frequency f′results in a deviation between an actual rotational speed R′ of themotor 120 and the ideal rotational speed R. Such deviation may bedescribed according to the following derivation:

$\begin{matrix}{R^{\prime} = \frac{C}{t^{\prime}}} \\{= \frac{C}{\left( {t \times \frac{f \pm {\Delta \; f}}{f}} \right)}} \\{= {\frac{C}{t} \times \frac{f}{f \pm {\Delta \; f}}}} \\{= {\frac{f}{f \pm {\Delta \; f}}R}} \\{= {{\frac{f \pm {\Delta \; f}}{f \pm {\Delta \; f}}R} - {\frac{{\pm \Delta}\; f}{f \pm {\Delta \; f}}R}}} \\{= {R \mp {\frac{\Delta \; f}{f \pm {\Delta \; f}}R}}}\end{matrix}$

(Equation 1), where R′ is the actual rotational speed that contains anerror due to the non-ideal (actual) clock frequency, and Δf is thedifference between the ideal frequency f and the actual frequency f′.

According to various embodiments, the control circuit 105 may beconfigured to detect and/or control the rotational speed of the motor120. The control circuit 105 may be connected to the motor 120 and maycomprise various circuits to perform various functions, such assampling, conversion, feedback control, counting, decoding, generating adrive signal, and computing. For example, according to variousembodiments, the control circuit 105 may comprise a compensation circuit140, a sampling circuit 145, the counter 150, a conversion circuit 155,a feedback control circuit 170, and a drive controller 175.

The calibration circuit 140 may be configured to determine andcompensate for errors in the motor system 100. For example, thecalibration circuit 140 may be configured to generate and transmit acompensation factor CF to the conversion circuit 155. The calibrationcircuit 140 may comprise any number of devices and/or componentssuitable for performing arithmetic functions, such as logic gates,discrete components (e.g., resistors and capacitors), transistors, andthe like.

For example, the calibration circuit 140 may be configured to compute afirst compensation factor CF, such as a compensated constant C′,according to the following:

$C^{\prime} = {\frac{f \pm {\Delta \; f}}{f}C}$

(Equation 2), where f is the ideal clock frequency, Δf is the differencebetween the ideal frequency f and the actual frequency f′, and C is theconstant.

The calibration circuit 140 may be further configured to compute asecond compensation factor CF, such as a change in constant ΔC,according to the following:

${\Delta \; C} = {{C^{\prime} - C} = {{{\frac{f \pm {\Delta \; f}}{f}C} - C} = {\frac{{\pm \Delta}\; f}{f}C}}}$

(Equation 3), where f is the ideal clock frequency, Δf is the differencebetween the ideal frequency f and the actual frequency f′, and C is theconstant, and C′ is the compensated constant.

The calibration circuit 140 may be further configured to transmit thecompensated constant C′ and/or the change in constant ΔC to theconversion circuit 155. For example, an output terminal of thecalibration circuit 140 may be connected to an input terminal of theconversion circuit 155.

The sampling circuit 145 may be configured to sample an input signal.For example, in one embodiment, the sampling circuit 145 may beconnected the output terminal of the comparator 135 and configured tosample a comparator output signal. In an alternative embodiment, thesampling circuit 145 may be connected to an output terminal of thezero-crossing point detection circuit 210 and configured to sample azero-crossing signal. The sampling circuit 145 may comprise any suitablecircuit for sampling a voltage of a continuous-time signal, holding thevalue for some period of time, and generating a discrete-time signalaccording to the sampled values, such as a conventional sample-and-holdcircuit.

The counter circuit 150 may be configured to measure a cycle time of themotor 120 and generate an actual counter value t′ that represents thelength of time for a specified rotation cycle of the motor 120. Forexample, the specified rotation cycle may be defined as one fullrotation cycle or a half rotation cycle. In one embodiment, a totalrotational period T (measured in seconds) of the motor 120 may bedescribed as: T=2t/f; where f is the ideal clock frequency and t is anideal counter value for a half rotation cycle of the motor 120. However,due to the error in the clock signal CLK, the total rotational period Tmay be described as: T=2t′/(f±Δf), where t′ is an actual counter value.Therefore, the actual counter value t′ is described as: t′=[(f±Δf)/f]t.

In an alternative embodiment, the total rotational period T (measured inseconds) of the motor 120 may be described as: T=t/f, where f is theideal clock frequency and t is an ideal counter value for a fullrotation cycle of the motor 120. However, due to the error in the clocksignal CLK, the total rotational period T may be described as:T=t′/(f±Δf), where t′ is an actual counter value.

In alternative embodiments, the equation used to describe the totalrotational period T may be based on the type of motor 120 that is used,as the mathematical relationship between the rotational period T,counter value t, and frequency f vary based on the type of motor.

In an exemplary embodiment, the counter circuit 150 may be connected toan output terminal of the sampling circuit 145 and generate the actualcounter value t′ according to the sampling circuit 145 outputs. Thecounter circuit 150 may be further configured to transmit the actualcounter value t′ to the conversion circuit 155. The counter circuit 150may comprise any suitable circuit for measuring periods, such as aconventional frequency counter.

The conversion circuit 155 may be configured to compute the actualrotational speed R′ of the motor 120. For example, the conversioncircuit 155 may be configured to compute the actual rotational speed R′according to the following:

$R^{\prime} = {\frac{C\; \prime}{t\; \prime} = {\frac{\left( {C \times \frac{f \pm {\Delta \; f}}{f}} \right)}{\left( {t \times \frac{f \pm {\Delta \; f}}{f}} \right)} = {\frac{C}{t} = R}}}$

(Equation 4), where the actual rotational speed R′ is measured inrevolutions per minute (RPM), C′ is the compensated constant, and t′ isthe actual counter value, f is the ideal clock frequency, and Δf is thedifference between the ideal clock frequency f and the actual clockfrequency f′. The conversion circuit 155 may transmit the computedactual rotational speed R′ to the feedback control circuit 170. Theconversion circuit 155 may comprise any suitable circuit and/or systemcapable of performing various arithmetic functions, such as thoserequired in Equation 4.

According to various embodiments, the control circuit 105 may furthercomprise a duty ratio calculation circuit 165 configured to receive apulse-width modulation input (PWM_IN) signal and compute a duty ratio DRof the PWM_IN signal. The duty ratio calculation circuit 165 maytransmit the duty ratio DR to a decoder 160 configured to decode theduty ratio DR, wherein the decoded duty ratio represents a targetrotational speed TRS (measured in RPM). In other words, the decoder 160converts the duty ratio DR into the target rotational speed TRS. Thedecoder 160 may be configured to transmit the target rotational speedTRS to the feedback control circuit 170.

The feedback control circuit 170 may be configured to receive varioussignals and feedback signals. For example, the feedback control circuit170 may be configured to receive the target rotational speed TRS and theactual detected rotational speed R′ as inputs. The feedback controlcircuit 170 may utilize the target rotational speed TRS and the actualrotational speed R′ to generate a drive PWM signal (PWM_DR).

According to various embodiments, the motor system 100 may furthercomprise a drive control circuit 175, a level shifter circuit 110, and adriver 115 that operate together to generate a desired electrical energyand supply the energy to the motor 120.

The drive control circuit 175 may be connected to an output terminal ofthe feedback control circuit 170, receive the drive PWM signal andgenerate a drive signal (drive current or drive voltage) according tothe drive PWM signal. The drive control circuit 175 may transmit thedrive signal to the level shifter circuit 110. According to variousembodiments, the drive control circuit 175 may comprise any circuitand/or system suitable for generating a drive current or a drive voltageaccording to an input signal, such as the drive PWM signal.

The level shifter circuit 110 may be configured to shift (increase ordecrease) the drive current or the drive voltage from the drive controlcircuit 175 to ensure compatibility with the driver 115 and generate ashifted drive signal. For example, the level shifter circuit 110 mayincrease the voltage of the signal from the drive control circuit 175 toa supply voltage Vcc in order to control transistors in the driver 115.The level shifter circuit 110 may comprise any circuit and/or systemsuitable for translating signals from one logic level or voltage domainto another.

The driver 115 may receive the shifted drive signal from the levelshifter circuit 110 and operate according to the shifted drive signal todrive and/or control the operation of the motor 120. The driver 115 maycomprise any circuit and/or system suitable for driving a motor. Forexample, the driver 115 may comprise a plurality of transistors arrangedto provide a desired driver architecture, such as an H-bridge driver ora 3-phase driver.

According to various embodiments, the control circuit 105 and/or thecalibration circuit 140 may be connected to and/or have access to amemory 130. The memory 130 may comprise any suitable memory type, forexample the memory 130 may comprise a non-volatile memory device. Thememory 130 may be used to store relevant values, such as the frequencydeviation Δf and the compensated constant C′.

According to various embodiments, the memory 130 may be configured tostore a binary number that represents the constant C and/or thecompensated constant C′. For example, the constant C, the compensatedconstant C′, and/or the change in constant ΔC may be represented by anumber of bits, such as 110101001110001110 (represented using 19 bits).The number of bits used to represent the constant C and/or thecompensated constant C′ may be based on the desired accuracy of thevariables, the specifications of the memory 130, and/or otherparticulars of the motor system 100.

According to various embodiments, the memory 130 may be configured tostore a variable portion C₁′ of the binary number. For example, if theconstant C and/or the compensated constant C′ is represented as1101010011100001110, then the memory 130 may store a signed 8-bitnumber, such as bit positions 8-16 (i.e., C₁′=010100111). The constant Cmay be multiplied by a step factor resulting in a range of values toaccount for possible values of the compensated constant C′ and thevariable portion C′.

According to various embodiments, various components, such as the motor120, the control circuit 105, the memory 130, and the like, may becapable of communicating with a test system (not shown). The test systemmay be implemented as a separate electronic device used to measurevarious characteristics of the motor system 100, such as voltage,current, frequency, and time. The test system may be further configuredto detect various output signals from the components and/or providespecific signal patterns (e.g., square waveforms) to the components thatare undergoing testing. The testing system may be further configured toprogram and/or erase the memory 130. The test system may be furtherconfigured to perform various calculations, such as calculating thecompensated constant C′ and/or the calculating the frequency deviationΔf. The test system may comprise any circuits and/or systems suitablefor measuring device characteristics such as voltage, current,frequency, and time. For example, the test system may comprise variousdetection circuits, such as a voltage detection circuit and a currentdetection circuit. The test system may further comprise various circuitsand/or systems suitable for performing various arithmetic functions. Forexample, the test system may comprise a logic circuit formed from logicgates, flip flops, and other combinational logic circuits configured toperform desired arithmetic functions, such as addition and/orsubtraction to compute the deviation frequency Δf and/or the compensatedconstant C′.

According to various operations, the methods for controlling the motor120 comprise calibrating the motor system 100 so that motor controland/or the rotation speed can be more accurately controlled and/ordetected. According to various operations, the methods for controllingthe motor 120 comprise removing and/or compensating for error introducedby the clock signal CLK.

According to various operations, the motor system 100 may operateaccording to three stages: a first stage (a test stage), a second stage(a calibration/initialization stage), and a third stage (an operationalstage). The first stage may comprise measuring the actual clockfrequency f′ and computing the frequency deviation (Δf). In someoperations, the first stage may further comprise calculating the changein constant ΔC. The second stage may comprise powering the motor system100, and calculating the compensated constant C′. The third stage maycomprise generating the actual counter value t′, calculating the actualrotational speed R′ and/or controlling the rotational speed R′.

In a first method, and referring to FIG. 3, the first stage may comprisemeasuring the actual clock frequency f′ (300), calculating the frequencydeviation Δf using the test system (not shown) (305), and storing thefrequency deviation value Δf in the memory 130 (310). The second stagemay comprise powering the motor system (315), retrieving the frequencydeviation value Δf from the memory 130 (320), calculating thecompensated constant C′ according to Equation 2 using the calibrationcircuit 140 (325), and transmitting the calculated compensated constantC′ to the conversion circuit 155 (330). The third stage may comprisecalculating the actual rotational speed R′ using the calculatedcompensated constant C′ and the conversion circuit 155 (345),controlling the rotational speed using the feedback control circuit 170,the drive control circuit 175, the level shifter circuit 110, the driver115, the duty ratio calculation circuit 165 and/or the decoder 160(350), measuring the cycle time via the counter 150 and generating theactual counter value t′ (340), and updating the rotational speed (345)according to the actual counter value t′.

In a second method, and referring to FIG. 4, the first stage maycomprise measuring the actual clock frequency f′ (400), calculating thefrequency deviation Δf using the test system (not shown) (405),calculating the compensated constant C′ according to Equation 2 usingthe test system (410), and storing the compensated constant C′ in thememory 130 (415). The second stage may comprise powering the motorsystem (420), retrieving the compensated constant C′ from the memory 130(425), and transmitting the calculated compensated constant C′ to theconversion circuit 155 (430). The third stage may comprise calculatingthe actual rotational speed R′ using the calculated compensated constantC′ and the conversion circuit 155 (440), controlling the rotationalspeed using the feedback control circuit 170, the drive control circuit175, the level shifter circuit 110, the driver 115, the duty ratiocalculation circuit 165 and/or the decoder 160 (445), measuring thecycle time via the counter 150 and generating the actual counter valuet′ (435), and updating the rotational speed (440) according to theactual counter value t′.

In a third method, and referring to FIG. 5, the first stage may comprisemeasuring the actual clock frequency f′ (500), calculating the frequencydeviation Δf using the test system (not shown) (505), calculating thechange in constant ΔC according to Equation 3 using the test system(510), and storing the change in constant ΔC in the memory 130 (515).The second stage may comprise powering the motor system (520),retrieving the change in constant ΔC from the memory 130 (525),computing the compensated constant C′ using the change in constant ΔC,(where C′=C+ΔC) using the calibration circuit 140 (530), andtransmitting the calculated compensated constant C′ to the conversioncircuit 155 (535). The third stage may comprise calculating the actualrotational speed R′ using the calculated compensated constant C′ and theconversion circuit 155 (545), controlling the rotational speed using thefeedback control circuit 170, the drive control circuit 175, the levelshifter circuit 110, the driver 115, the duty ratio calculation circuit165 and/or the decoder 160 (550), measuring the cycle time via thecounter 150 and generating the actual counter value t′ (540), andupdating the rotational speed (545) according to the actual countervalue t′.

In a fourth method, and referring to FIG. 6, the first stage maycomprise measuring the actual clock frequency f′ (600), calculating thefrequency deviation Δf using the test system (not shown) (605),calculating the compensated constant C′ according to Equation 2 usingthe test system (610), and extracting the variable portion C₁′ from thecompensated constant C′ and storing the variable portion C₁′ in thememory 130 (615). The second stage may comprise powering the motorsystem (620), retrieving the variable portion of the compensatedconstant C₁′ from the memory 130 (625), reconstructing the compensatedconstant C′ based on the variable portion C₁′ using the calibrationcircuit 140, and transmitting the calculated compensated constant C′ tothe conversion circuit 155 (630). The third stage may comprisecalculating the actual rotational speed R′ using the calculatedcompensated constant C′ and the conversion circuit 155 (640),controlling the rotational speed using the feedback control circuit 170,the drive control circuit 175, the level shifter circuit 110, the driver115, the duty ratio calculation circuit 165 and/or the decoder 160(645), measuring the cycle time via the counter 150 and generating theactual counter value t′ (635), and updating the rotational speed (640)according to the actual counter value t′.

In the foregoing description, the technology has been described withreference to specific exemplary embodiments. The particularimplementations shown and described are illustrative of the technologyand its best mode and are not intended to otherwise limit the scope ofthe present technology in any way. Indeed, for the sake of brevity,conventional manufacturing, connection, preparation, and otherfunctional aspects of the method and system may not be described indetail. Furthermore, the connecting lines shown in the various figuresare intended to represent exemplary functional relationships and/orsteps between the various elements. Many alternative or additionalfunctional relationships or physical connections may be present in apractical system.

The technology has been described with reference to specific exemplaryembodiments. Various modifications and changes, however, may be madewithout departing from the scope of the present technology. Thedescription and figures are to be regarded in an illustrative manner,rather than a restrictive one and all such modifications are intended tobe included within the scope of the present technology. Accordingly, thescope of the technology should be determined by the generic embodimentsdescribed and their legal equivalents rather than by merely the specificexamples described above. For example, the steps recited in any methodor process embodiment may be executed in any order, unless otherwiseexpressly specified, and are not limited to the explicit order presentedin the specific examples. Additionally, the components and/or elementsrecited in any apparatus embodiment may be assembled or otherwiseoperationally configured in a variety of permutations to producesubstantially the same result as the present technology and areaccordingly not limited to the specific configuration recited in thespecific examples.

Benefits, other advantages and solutions to problems have been describedabove with regard to particular embodiments. Any benefit, advantage,solution to problems or any element that may cause any particularbenefit, advantage or solution to occur or to become more pronounced,however, is not to be construed as a critical, required or essentialfeature or component.

The terms “comprises”, “comprising”, or any variation thereof, areintended to reference a non-exclusive inclusion, such that a process,method, article, composition or apparatus that comprises a list ofelements does not include only those elements recited, but may alsoinclude other elements not expressly listed or inherent to such process,method, article, composition or apparatus. Other combinations and/ormodifications of the above-described structures, arrangements,applications, proportions, elements, materials or components used in thepractice of the present technology, in addition to those notspecifically recited, may be varied or otherwise particularly adapted tospecific environments, manufacturing specifications, design parametersor other operating requirements without departing from the generalprinciples of the same.

The present technology has been described above with reference to anexemplary embodiment. However, changes and modifications may be made tothe exemplary embodiment without departing from the scope of the presenttechnology. These and other changes or modifications are intended to beincluded within the scope of the present technology, as expressed in thefollowing claims.

1. A control circuit connected to a motor, comprising: a calibrationcircuit having access to a memory and configured to: compute a deltaclock frequency value according to a measured clock frequency and anideal clock frequency; and compute a compensated value according to thedelta clock frequency; and a conversion circuit connected to thecalibration circuit and configured to: receive the compensated value;and compute a rotational speed of the motor according to the compensatedvalue; wherein the control circuit operates the motor according to thecomputed rotational speed.
 2. The control circuit according to claim 1,wherein the control circuit further comprises a counter circuitconfigured to measure a cycle time of the motor and generate a countervalue that represents the cycle time.
 3. The control circuit accordingto claim 2, wherein the counter circuit is connected to an inputterminal of the conversion circuit and configured to transmit thecounter value to the conversion circuit.
 4. The control circuitaccording to claim 3, wherein the conversion circuit is furtherconfigured to compute the rotational speed of the motor based on thecounter value.
 5. The control circuit according to claim 1, wherein thecontrol circuit further comprises a feedback control circuit connectedto an output terminal of the conversion circuit.
 6. The control circuitaccording to claim 5, wherein the control circuit is further configuredto receive a pulse width modulation input signal and generate a targetrotational speed according to the pulse width modulation input signal.7. The control circuit according to claim 6, wherein the feedbackcontrol circuit is configured to: receive the target rotational speedand the computed rotational speed; and generate a drive signal accordingto the target rotational speed and the computed rotational speed.
 8. Thecontrol circuit according to claim 1, wherein the compensated value isrepresented as a binary number having a predetermined number of bits andthe memory stores the predetermined number of bits.
 9. The controlcircuit according to claim 1, wherein the compensated value isrepresented as a binary number having a predetermined number of bits andthe memory stores only a portion of the bits.
 10. A method for operatinga motor system with a motor, comprising: determining a frequencydeviation comprising: measuring an actual clock frequency; and computinga change in frequency from an ideal clock frequency using the actualclock frequency; calibrating the motor system comprising: computing acompensated value according to the change in frequency, wherein thecompensated value is represented as a binary number further having apredetermined number of bits; and computing a rotational speed accordingto the compensated value; and controlling a rotational speed of themotor according to the computed rotational speed.
 11. The methodaccording to claim 10, further comprising measuring a length of time ofa cycle of the motor.
 12. The method according to claim 10, whereincontrolling the rotational speed of the motor further comprisesgenerating a feedback signal according to the computed rotational speedand controlling the rotational speed of the motor according to thefeedback signal
 13. The method according to claim 10, whereincalibrating the motor system comprises computing the compensated valueaccording to a predetermined constant and the ideal frequency.
 14. Themethod according to claim 10, further comprising storing thepredetermined number of bits to a memory.
 15. The method according toclaim 10, further comprising storing only a portion of the bits to amemory.
 16. A system having a motor, comprising: a clock generatorcircuit having an ideal frequency and an actual frequency; a memory; acontrol circuit connected to a motor and having access to the memory,wherein the control circuit comprises: a calibration circuit havingaccess to the memory and configured to: compute a delta clock frequencyvalue according to the actual frequency and the ideal frequency; andcompute a compensated value according to the delta clock frequency and apredetermined constant; a counter configured to measure a cycle time ofthe motor and generate a counter value according to the measured cycletime; and a conversion circuit connected to the calibration circuit andconfigured to: receive the compensated value; and compute a rotationalspeed of the motor according to the compensated value and the countervalue; wherein the control circuit operates the motor according to thecomputed rotational speed.
 17. The system according to claim 16, whereinthe control circuit further comprises a feedback control circuitconnected to an output terminal of the conversion circuit.
 18. Thesystem according to claim 16, wherein the control circuit is furtherconfigured to receive a pulse width modulation input signal and generatea target rotational speed according to the pulse width modulation inputsignal.
 19. The system according to claim 16, wherein the feedbackcontrol circuit is configured to: receive the target rotational speedand the computed rotational speed; and generate a drive signal accordingto the target rotational speed and the computed rotational speed. 20.The system according to claim 16, further comprising a feedback circuitconnected to the motor and configured to detect an actual rotationalspeed of the motor.